Polymeric articles may be formed by injecting highly chemically reactive liquid constituents into a mold where they polymerize in situ. Before molding, the constituents are nucleated with pressurized gas in the form of minute bubbles. After injection, the gas expands promoting mold fill-out and microcellularity (with resultant reduced density) in the polymerized article.
This invention relates to a method and means for monitoring the degree of gas entrainment in such constituents prior to molding. More particularly, the invention relates to the controlled volumetric expansion of reactive constituent samples to determine the amount of gas actually entrained therein relative to the desired amount.
Reaction injection molding (RIM) generally pertains to injecting highly chemically reactive liquid constituents into a mold wherein they rapidly polymerize to form a desired article. Relatively large, structural automotive parts such as automobile fascia and quarter panels have been formed from reaction injection molded thermosetting urethanes. In urethane RIM systems, a catalyzed stream of liquid polyol is impingement mixed with a stream of isocyanate under high pressure. We have found it expedient to impregnate at least one of the constituents with a pressurized gas. Herein, the process of introducing an agent into a constituent prior to molding which is gaseous and expands in the mold may be referred to as nucleation.
The gas is entrained in the form of minute bubbles introduced, for example, through a microporous diffusion element as taught in U.S. Pat. No. 4,157,427 assigned to the assignee hereof, or by whipping a pressurized gas blanket into a bulk holding tank of constituent. The entrained gas expands as pressure on the constituents is relieved after mold injection. The gas expansion helps fill out the mold, promote uniform part density, and eliminate sink marks in thick mold sections. The urethane parts so produced are dense microcellular foams with smooth, paintable surfaces. For automotive applications, a cured part density of about 90% of the density of the unblown urethane is desirable.
It is often desirable to reinforce RIM articles with fiberglass or other particulate fillers. The fillers are preferably slurried in amounts up to 50 weight percent with agitated precursor constituents. Gas is entrained in the slurries as described above.
In order to predictably and consistently reaction injection mold high quality microcellular parts, it is necessary to know the degree of gas entrained in the precursor constituents prior to molding. One method of doing this has been to periodically monitor the specific gravity of the constituents with a device such as a Dynatrol.RTM.. The specific gravity of a constituent generally decreases in proportion to the amount of gas or blowing agent present. Another means of measuring the amount of entrained gas is set forth in U.S. Pat. No. 4,050,896. In that method, the volumetric flow rate of a gas-charged reaction component is measured at a first pressure level. The component is then brought to a second, lower pressure level and the volumetric flow rate is measured again. The differential flow rate is a function of the amount of entrained gas.
Neither of the above-described methods for measuring the amount of entrained gas, nor any other method of which applicants are aware, is adaptable for use with filled systems. Small variations in the amount of filler present can cause large fluctuations in the specific gravity of the liquid constituent in which they are entrained. Moreover, the relatively sensitive and costly instruments used to measure flow rate and specific gravity may not tolerate the presence of abrasive filler particles. Moreover, these methods are relatively complex compared to the method which is the subject of this invention. While our invention represents the only known practical method and apparatus for measuring gas entrainment in filled RIM systems, it is also applicable to unfilled RIM systems and represents a substantial improvement in the art.
Thus it is an object of this invention to provide an improved method of monitoring gas entrainment in a pressurized liquid precursor constituent for molding polymeric articles, particularly where the precursor constituent contains abrasive filler particles. A more specific object is to provide a method of expanding a sample of a liquid RIM precursor constituent from a first volume to a different second volume, measuring the sample pressure at the second volume and comparing the measured pressure to the pressure corresponding to the desired degree of gas entrainment. A further object of the method is to control the addition or withdrawal of gas to or from a RIM precursor constituent on the basis of the measured pressure of a sample thereof after its controlled volumetric expansion.
Another object is to provide an improved, relatively inexpensive and wear-resistant means of monitoring gas entrainment in a chemically reactive liquid precursor constituent for molding polymeric articles by injecting the constituent into a mold and polymerizing it therein. A more specific object is to provide means for expanding a sample of a RIM precursor constituent containing entrained gas, for measuring the pressure of the expanded sample, and for comparing the measured pressure to the pressure corresponding to the ideal amount of entrained gas in the constituent. Another object is to provide means to use such pressure measurement and comparison to automatically control the addition or withdrawal of gas from the constituent. Another object is to provide a means of monitoring gas entrainment in a particle filled RIM precursor constituent.